1. Field of the Invention
The present invention relates to a reinforced material for an automobile connecting rod providing improved tensile strength and bending strength.
2. Description of the Related Art
To achieve high efficiency in an engine, the parts undergoing reciprocating and rotating motion at a high speed, notably the connecting rods linking the pistons and the crank shaft, need to have low weight, good abrasion resistance, and high strength. The strength requirement is important because connecting rods must bear a compressive force, a tensile force, and complex bending forces created by the crank motion.
Conventional reinforced materials for connecting rods are made by utilizing as a reinforcing core a bundle of stainless steel wire, each having a diameter of about 25 .mu.m. The bundle is typically placed into a heat-resistant tube such as a quartz tube and heated to about 700.degree. C. for ten minutes to promote partial fusion of the stainless steel wire. The core is then generally placed into a mold such as a squeeze casting mold with a molten aluminum alloy as a matrix metal and the material is cast under a force of 500 to 2000 kgf/cm.sup.2. The molded article is then machined to form the connecting rod.
FIGS. 5, 6, and 7 are micrographs from a scanning electron microscope, or SEM, of connecting rod material made by this conventional process. FIG. 5 shows that the reinforcing fibers are relatively equally distributed in the metal matrix, and FIG. 6 shows that the reinforcing fibers are hexagonal-shaped. FIG. 7 shows the fibers to be partially fused due to the heating step discussed above.
The prior art method is undesirable because the stainless steel wire loses strength when the temperature is over 700.degree. C. The aluminum in the metal matrix and the iron in the reinforcing stainless steel wire may react at these temperatures to form an intermetallic Fe.sub.m Al.sub.n compound (such as Fe.sub.2 Al.sub.5), which is a fragile material. This occurs because the solubility of steel in aluminum is as low as 0.01 to 0.12 wt. % at temperatures of 275.degree.-600.degree. C. As this intermetallic compound becomes thicker around the stainless steel fibers, the tensile strength and fracture elongation is reduced.
As a general rule, therefore, the formation of this intermetallic compound due to interfacial reaction between the metal matrix and the reinforcing fibers detrimentally affects the properties of the composite material. Linear reinforcing fibers are often used to try to minimize this reaction, but while such fibers have high tensile strength, the fracture strain is low.
Therefore, the need exists for a method of reinforcing a metal matrix while minimizing the formation of a fragile intermetallic compound at the interface between the reinforcing fibers and the metal matrix.